Optical navigation system and method of estimating motion with optical lift detection

ABSTRACT

An optical navigation system and method of estimating motion uses a plate with an aperture, a photodetector and an optical system for optical lift detection. The optical system is configured to direct an input light to a target surface through the aperture of the plate and to direct the input light reflected from the target surface and transmitted back through the aperture of the plate toward the photodetector to be detected by the photodetector for lift detection.

BACKGROUND OF THE INVENTION

Optical navigation systems operate to estimate movements between theoptical navigation systems and target surfaces to perform trackingoperations. An optical navigation system uses a light source, such as alight-emitting diode (LED) or a laser diode, and an image sensor tosuccessively capture frames of image data of a target surface. Theoptical navigation system compares the successive image frames andestimates the relative movements between the optical navigation systemand the target surface based on the comparison between the current imageframe and a previous image frame. The comparison is based on detectingand computing displacements of features in the captured frames of imagedata. For laser-based navigation systems, these features are usuallyinterference images produced by a laser spot impinging on the targetsurface.

Optical navigation systems are commonly used in optical computer mice totrack the movements of the mice relative to the surfaces on which themice are manually manipulated. In order to perform the trackingoperation properly, an optical mouse needs to be on the target surfacesince errors are introduced when the distance between the image sensorof the optical navigation system and the target surface is significantlyincreased, i.e., when the optical mouse has been lifted from the targetsurface. In an optical mouse with a laser-based navigation system, theoptical interference images used for motion estimation grow as thedistance between the image sensor of the system and the target surfaceis increased. Consequently, the navigation system can still beresponsive after the optical mouse has been lifted from the targetsurface, which means erroneous motion estimates will be made by thenavigation system.

The optical characteristics of the interference images do not allow ameaningful solution to the issue of lift detection using the imagesensor of the optical navigation system. Thus, a separate sensingmechanism such as a mechanical switch is needed to detect when theoptical mouse is lifted from a target surface.

Although conventional lift detection mechanisms work well for theirintended purpose, there is a need for a non-complex and low cost liftdetection mechanism for use in optical navigation systems.

SUMMARY OF THE INVENTION

An optical navigation system and method of estimating motion uses aplate with an aperture, a photodetector and an optical system foroptical lift detection. The optical system is configured to direct aninput light to a target surface through the aperture of the plate and todirect the input light reflected from the target surface and transmittedback through the aperture of the plate toward the photodetector to bedetected by the photodetector. The input light detected at thephotodetector indicates that the optical navigation system has not beenlifted from the target surface. Conversely, no light (except fornegligible light) detected at the photodetector indicates that theoptical navigation system has been lifted from the target surface.

An optical navigation system in accordance with an embodiment of theinvention comprises a light source, an image sensor and a lift detectionunit. The light source is configured to generate light. The image sensoris positioned to receive a first portion of the light reflected from atarget surface. The image sensor is configured to generate frames ofimage data in response to the received first portion of the light formotion estimation. The lift detection unit comprises a plate with anaperture, a photodetector and an optical system. The photodetector ispositioned over the plate. The optical system is positioned over theplate to receive a second portion of the light from the light source.The optical system is configured to direct the second portion of thelight through the aperture of the plate. The optical system is furtherconfigured to direct the second portion of the light reflected from thetarget surface and transmitted back through the aperture of the platetoward the photodetector to be detected by the photodetector.

An optical navigation system in accordance with an embodiment of theinvention comprises a plate with an aperture, a photodetector and anoptical system. The photodetector and the optical system are positionedover the plate. The optical system comprises first, second and thirdprisms. The first prism is positioned to receive an input light. Thefirst prism is configured and orientated to refract the input lightthrough the aperture of the plate. The second prism is positioned toreceive the input light reflected from a target surface and transmittedback through the aperture of the plate. The second prism is configuredand orientated to refract the input light to a predefined direction. Thethird prism is positioned to receive the input light from the secondprism. The third prism is configured to refract the input light towardthe photodetector to be detected by the photodetector.

A method of estimating motion in accordance with an embodiment of theinvention comprising generating light, directing a first portion of thelight toward a target surface, receiving the first portion of the lightreflected from the target surface at an image sensor for motionestimation, directing a second portion of the light toward the targetsurface through an aperture positioned over the target surface,directing the second portion of the light reflected from the targetsurface and transmitted back through the aperture toward aphotodetector, and receiving the second portion of the light at thephotodetector.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrated by way of example of theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an optical navigation system included in an opticalcomputer mouse in accordance with an embodiment of the invention.

FIG. 2 is a diagram of the optical navigation system in accordance withan embodiment of the invention.

FIG. 3 is a perspective view of a lower plate and an optical system ofthe optical navigation system in accordance with an embodiment of theinvention.

FIG. 4 is a diagram of the optical navigation system of FIG. 2, showingoptical paths of light through the optical navigation system when theoptical computer mouse has not been lifted.

FIG. 5 is a diagram of the optical navigation system of FIG. 2, showingoptical paths of light through the optical navigation system when theoptical computer mouse has been lifted.

FIG. 6 is a process flow diagram of a method of estimating motion inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION

With reference to FIG. 1, an optical navigation system 100 in accordancewith an embodiment of the invention is described. As shown in FIG. 1,the optical navigation system 100 is included in an optical computermouse 102, which is connected to a computer 104. In this implementation,the optical navigation system 100 is used to track the movements of theoptical mouse 102 as the optical mouse is manipulated over a targetsurface 106, which may be a glass surface or other optically transparentsurface, by a user to control a cursor displayed on the computer 104.However, in other implementations, the optical navigation system 100 canbe used in different products for various tracking applications. Asdescribed in detail below, the optical navigation system 100 includes anoptical lift detection feature to optically detect when the opticalmouse 102 has been lifted from the surface in the Z direction. Theoptical lift detection feature of the optical navigation system 100 canbe implemented using low cost components without the requirement ofcritical alignment.

Turning now to FIG. 2, various components of the optical navigationsystem 100 are shown. FIG. 2 is a sectional view of the opticalnavigation system 100. As shown in FIG. 2, the optical navigation system100 includes a navigation unit 210 and a lift detection unit 212. Theprimary function of the navigation unit 210 is to capture frames ofimage data for motion estimation. The primary function of the liftdetection unit 212 is to detect when the optical mouse 102 with theoptical navigation system 100 has been lifted from the target surface106.

The navigation unit 212 includes a light source 214, an optical guidestructure 216 and an image sensor 218. The light source 214 generateslight, which will be used for both motion estimation and lift detection.In this embodiment, the light source 214 is a laser device, such as avertical-cavity surface-emitting laser (VCSEL), which generates coherentlight. However, in other embodiments, the light source 214 may be alight emitting diode or any other light emitting device. The lightsource 214 is positioned to emit light along the X direction into theoptical guide structure 216.

The optical guide structure 216 is an optically transparent structureconfigured to direct some of the light received from the light source214 to an imaging area 220 of the target surface 106. The optical guidestructure 216 is also designed to receive the light reflected off theimaging area 220 of the target surface 106 to transmit the reflectedlight to the image sensor 218. Furthermore, the optical guide structure216 is designed to transmit some of the light received from the lightsource 214 to the lift detection unit 212, which uses this light todetect when the optical mouse 102, or any other device, with the opticalnavigation system 100 has been lifted from the target surface 106. Inparticular, the optical guide structure 216 is designed to transmit someof the light from the light source 214 to the lift detection unit 212such that the light is mostly traveling along the X direction.

The optical guide structure 216 may be structurally shaped in any numberof configurations, as long as the optical guide structure can opticallyperform the above-described tasks. In the illustrated embodiment, theoptical guide structure 216 includes an input port 222 to receive thelight emitted from the light source 214 so that the received light istransmitted into the optical guide structure along the X direction. Theinput port 222 of the optical guide structure 216 has a convex surfaceto focus the received light from the light source 214. The optical guidestructure 216 also includes an intermediate section 224, whichinternally reflects the light from the input port 222 so that the lightis maintained along the X direction but closer to the bottom of theoptical guide structure 216, i.e., closer to the target surface 106 onwhich the optical computer mouse 102 is being operated. The intermediatesection 224 of the optical guide structure 216 includes an upper surface226, which reflects the light from the input port 222 downward along theZ direction, and a lower surface 228, which reflects the light from theupper surface 226 back along the X direction. The optical guidestructure 216 also includes an output section 230, which separates thelight from the lower surface 228 so that some of the light can be usedfor motion estimation and some of the light can be used for liftdetection. The output section 230 includes a reflective surface 232,which reflects a portion of the light from the lower surface 228downward along the Z direction toward the imaging area 220 of the targetsurface 106 for use in motion estimation. The output section 230 alsoincludes an output port 234 to output a portion of the light from thelower surface 228 along the X direction toward the lift detection unit212 for use in lift detection.

The image sensor 218 is positioned above the optical guide structure 216to receive the light reflected off the imaging area 220 of the targetsurface 106 to capture frames of image data of the target surface.Specifically, the image sensor 218 is positioned over the reflectivesurface 232 of the optical guide structure 216 to receive the lightreflected from the imaging area 220 of the target surface 106. The imagesensor 218 includes an array of photosensitive pixel elements (notshown), which generate image signals in response to light incident onthe elements. As an example, the image sensor 218 may be acharged-coupled device (CCD) image sensor or a complementary metal oxidesemiconductor (CMOS) image sensor. The number of photosensitive pixelelements included in the image sensor 218 may vary depending on at leastperformance requirements of the optical navigation system 100 withrespect to optical motion estimation. As an example, the image sensor218 may include a 30×30 array of active photosensitive pixel elements.

The lift detection unit 212 of the optical navigation system 100includes a lower plate 236, an upper plate 238, an optical system 240and a photodetector 242. The upper plate 238 is positioned over thelower plate 236 such that there is a predefined distance between theplates. The lower and upper plates 236 and 238 are positioned next tothe output port 234 of the optical guide structure 216. Both the lowerand upper plates 236 and 238 are orientated to be parallel along the Xdirection. As best shown in FIG. 3, which is a perspective view of thelower plate 236 and the optical system 240, the lower plate includes anaperture 244. In the illustrated embodiment, the aperture 244 of thelower plate 236 is rectangular in shape. However, in other embodiments,the aperture 244 of the lower plate 236 may have a different shape, suchas a circular shape. The lower and upper plates 236 and 238 areoptically opaque such that light is block by the plates. Thus, only wayfor light to pass through the lower plate 236 is through the aperture244 of the lower plate. The lower and upper plates 236 and 238 of thelift detection unit 212 may be attached to the housing (not shown) ofthe optical computer mouse 102.

The optical system 240 is positioned between the lower and upper plates236 and 238 to receive the light emitted from the output port 234 of theoptical guide structure 216. The optical system 240 is configured todirect the received light downward through the aperture 244 of the lowerplate 236 at an angle greater than zero with respect to the X axis,which is parallel to the lower plate 236. The light transmitted throughthe aperture 244 of the lower plate 236 is then reflected off the targetsurface 106. The optical system 240 is further configured to receive thelight reflected off the target surface and transmitted back through theaperture 244 of the lower plate 236, and to direct that light to thephotodetector 242, which in the illustrated embodiment is located abovethe upper plate 238 but not directly over the upper plate. The upperplate 238 is used to prevent any unwanted light from reaching thephotodetector 242.

The optical system 242 includes three prisms 246, 248 and 250, which arepositioned between the lower and upper plates 236 and 238. In theillustrated embodiment, the prisms 246, 248 and 250 are triangularprisms with three major sides. Specifically, in the illustratedembodiment, the prisms 246, 248 and 250 are right triangular prisms.Thus, the cross-section of the prisms 246, 248 and 250 is righttriangular in shape. In the illustrated embodiment, the non-right angledcorners of the prisms 246, 248 and 250 are angled at forty-five degrees(45%). However, in other embodiments, these non-right angled corners ofthe prisms 246, 248 and 250 may be angled at different degrees.Furthermore, in other embodiments, the prisms 246, 248 and 250 may notbe right triangular prisms, and may even not be triangular prisms.

The first prism 246 is positioned between the optical guide structure216 and the aperture 244 of the lower plate 236. The first prism 246 isorientated such that its right angled corner is at the bottom near thelower plate 236 and the hypotenuse side of the prism 246 is facing theoutput port 234 of the optical guide structure 216 to receive lightemitted from the output port. Thus, the first prism 246 refracts thelight emitted from the output port 234 of the optical guide structure216 downward at an angle toward the aperture 244 of the lower plate 236.

The second prism 248 is positioned such that the aperture 244 of thelower plate 236 is located between the first prism 246 and the secondprism 248. The second prism 248 is orientated such that its right angledcorner is at the bottom near the lower plate 236 and the hypotenuse sideof the prism 248 is facing away from the output port 234 of the opticalguide structure 216. The second prism 248 is positioned to receive thelight from the first prism, which has been reflected off the targetsurface 106 and transmitted back through the aperture 244 of the lowerplate 236. The second-prism 248 refracts the received light so that therefracted light propagates along the X direction toward the third prism250.

The third prism 250 is positioned such that the second prism 248 islocated between the aperture 244 of the lower plate 236 and the thirdprism 250. The third prism 250 is orientated such that its right angledcorner is at the top near the upper plate 238 and the hypotenuse side ofthe prism 250 is facing away from the output port 234 of the opticalguide structure 216. In addition, the third prism 250 is located suchthat most of the hypotenuse side of the prism 250 extends out from underthe upper plate 238. The third prism 250 is positioned to receive therefracted light from the second prism 248. The third prism 250 refractsthe received light upward so that the refracted light propagates alongthe Z direction toward the photodetector 242.

The photodetector 242 is positioned over the third prism 250 to receivethe light refracted from the third prism 250. The photodetector 242 maybe any type of a light sensing device, such as a photodiode or otherlight intensity sensor. The photodetector 242 is configured to generatean electrical signal in response to incident light. As explained below,when the optical computer mouse 102 has not been lifted beyond apredefined height from the target surface 106, the photodetector 242will receive light from the optical system 240. However, when theoptical computer mouse 102 has been lifted beyond the predefined heightfrom the target surface 106, the photodetector 242 will not receive anylight from the optical system 240. Thus, the electrical signal generatedby the photodetector 242 in response to incident light can be used todetect when the optical computer mouse 102 has been “lifted” from thetarget surface 106. In addition, for a more robust system, a presetthreshold for optical power received by the photodetector 242 isdefined, which will eliminate any error signal generated by stray light.The predefined height that defines when the optical computer mouse 106has been “lifted” from the target surface 106 can be adjusted by varyingthe distances between the prisms 246, 248 and 250 of the lift detectionunit 212.

The operation of the optical navigation system 100 in accordance with anembodiment of the invention is described with reference to FIGS. 4 and5. FIG. 4 shows optical paths of light through the optical navigationsystem 100 when the optical computer mouse 102 has not been liftedbeyond the predefined height from the target surface 106. The lightemitted from the light source 214 is transmitted along the X directioninto the optical guide structure 216 at the input port 222. The light isthen internally reflected off the upper surface 226 of the optical guidestructure 216 downward along the Z direction. The light is then againinternally reflected off the lower surface 228 of the optical structure216 so that the reflected light propagates again along the X direction.The light is then divided into first and second portions of light. Thefirst portion of light is reflected off the reflective surface 232 ofthe optical guide structure 216 downward toward the imaging area 220 ofthe target surface 106. This light is then reflected from the targetsurface 106 and received by the image sensor 218 to produce frames ofimage data for motion estimation. However, the second portion of lightis transmitted through output port 234 of the optical guide structure216 toward the lift detection unit 212.

The light emitted from the output port 234 of the optical guidestructure 216 is then refracted by the first prism 246 of the opticalsystem 240 of the lift detection unit 212 downward through the aperture244 of the lower plate 236 to the target surface 106. The light is thenreflected upward from the target surface 106. Due to the close proximityof the target surface 106 to the lower plate 236, the reflected light istransmitted back through the aperture 244 toward the second prism 248 ofthe lift detection unit 212. The light is then refracted by the secondprism 248 toward the third prism 250 of the lift detection unit 212along the X direction. At the third prism 250, the light is refractedupward along the Z direction toward the photodetector 242. The light isthen detected by the photodetector 242, which indicates that thecomputer optical mouse 102 has not been “lifted” from the target surface106.

FIG. 5 shows optical paths of light through the optical navigationsystem 100 when the optical computer mouse 102 has been lifted beyondthe predefined height from the target surface 106. The light emittedfrom the light source 214 travels through the optical guide structure216 in the same manner as described above. Thus, a first portion oflight is reflected off the target surface 106 and received at the imagesensor 218 for motion estimation. Furthermore, a second portion of lightis emitted from the output port 234 of the optical guide structure 216.The light emitted from the output port 234 is again refracted by thefirst prism 246 of the lift detection unit 212 downward through theaperture 244 of the lower plate 236 to the target surface 106. The lightis then reflected upward from the target surface 106. However, now thedistance between the target surface 106 and the lower plate 236 has beenincreased since the optical computer mouse 102 has been lifted beyondthe predefined height. Thus, the reflected light from the target surface106 is not transmitted back through the aperture 244 of the lower plate236. Rather, the reflected light from the target surface 106 is blockedby the lower plate 236. Thus, the reflected light from the targetsurface 106 does not reach the photodetector 242. Consequently, no light(except maybe negligible light) is detected by the photodetector 242,which indicates that the computer optical mouse 102 has been “lifted”from the target surface 106.

The lift detection technique of the optical navigation system 106 isindependent of the materials and characteristics of the target surface106 since it only needs to detect the intensity of light reaching thephotodetector 242. The lift detection sensitivity of the opticalnavigation system 100 is dependent on the size of the aperture 244 ofthe lower plate 236 of the lift detection unit 212. Thus, the opticalnavigation system 100 can be adjusted to be more sensitive by using asmaller aperture in the lower plate 236 of the lift detection unit 212.

A method of estimating motion in accordance with an embodiment of theinvention is described with reference to a process flow diagram of FIG.6. At block 602, light is generated. At block 604, a first portion ofthe light is directed toward a target surface. At block 606, the firstportion of the light reflected from the target surface is received at animage sensor for motion estimation. At block 608, a second portion ofthe light is directed toward the target surface through an aperturepositioned over the target surface. At block 610, the second portion ofthe light reflected from the target surface and transmitted back throughthe aperture is directed toward a photodetector. At block 612, thesecond portion of the light is received at the photodetector for liftdetection.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The scope of theinvention is to be defined by the claims appended hereto and theirequivalents.

1. A method of estimating motion comprising: generating light;separating said light into first and second portions of said light;directing said first portion of said light toward a first region of atarget surface; receiving said first portion of said light reflectedfrom said first region of said target surface at an image sensor formotion estimation; directing said second portion of said light toward asecond region of said target surface through an aperture positioned oversaid target surface, said first and second regions being differentregions of said target surface; directing said second portion of saidlight reflected from said second region of said target surface andtransmitted back through said aperture toward a photodetector; andreceiving said second portion of said light at said photodetector forlift detection.
 2. The method of claim 1 wherein said directing saidsecond portion of said light toward said target surface and saiddirecting said second portion of said light reflected from said surfaceand transmitted back through said aperture toward said photodetector areperformed using an optical system positioned over said aperture, saidoptical system comprising: a first prism positioned to receive saidsecond portion of said light, said first prism being configured andorientated to refract said second portion of said light through saidaperture toward said target surface; a second prism positioned toreceive said second portion of said light reflected from said targetsurface and transmitted back through said aperture, said second prismbeing configured and orientated to refract said second portion of saidlight along a predefined direction; and a third prism positioned toreceive said second portion of said light from said second prism, saidthird prism being configured to refract said second portion of saidlight toward said photodetector.
 3. The method of claim 2 wherein atleast one of said first, second and third prisms is a triangular prism.4. The method of claim 2 wherein said separating said light into saidfirst and second portions of said light includes separating said lightinto said first and second portions of said light using an optical guidestructure including a reflective surface, said optical guide structurebeing configured to manipulate said light toward said reflective surfaceso that only said first portion of said light strikes said reflectivesurface such that said first portion of said light is reflected fromsaid reflective surface toward said target surface and said secondportion of said light is transmitted to said first prism of said opticalsystem without being reflected by said reflective surface.
 5. The methodof claim 2 wherein at least one of said first, second and third prismsis a right triangular prism.
 6. The method of claim 2 wherein said firstand second prisms are positioned over a plate that includes saidaperture such that said aperture of said plate is located between saidfirst and second prisms.
 7. The method of claim 6 wherein said thirdprism is positioned over said plate such that said second prism islocated between said aperture of said plate and said third prism.
 8. Themethod of claim 1 wherein said generating said light includes generatingsaid light source from a light-emitting diode or a laser device.
 9. Themethod of claim 1 wherein said directing said second portion of saidlight toward said target surface includes transmitting said secondportion of said light through a first optical element of an opticalsystem to direct said second portion of said light through said aperturetoward said target surface, and wherein said directing said secondportion of said light reflected from said target surface and transmittedback through said aperture toward said photodetector includessuccessively transmitting said second portion of said light reflectedfrom said target surface through a second optical element and a thirdoptical element of said optical system to successively direct saidsecond portion of said light reflected from said target surface towardsaid photodetector.
 10. The method of claim 9 wherein each of saidfirst, second and third optical elements is a prism.